An MRI apparatus is characterized in that high tissue-contrast resolution can be obtained, and desired tomograms can be obtained; and, such an apparatus has no invasiveness, for example. The application of an MRI apparatus, having such characteristics, not only to diagnostic imaging, but also to interventional MRI, such as is used in connection with the guiding of a biopsy needle, the monitoring of therapy, the guiding of an endoscope and a catheter, and the like, has been drawing increased attention. In interventional MRI, a device, such as a biopsy needle and a catheter, is inserted in the body of the object to be examined, while an operator (a doctor) observes the position of the device using MR images. That is, the device in the object, the position of which changes with time, is continuously imaged, and a plurality of thus obtained images are sequentially displayed on a display, whereby the doctor can guide the device to a lesion during surgery.
On the other hand, with regard to an MRI apparatus, a technology for shortening the imaging time utilizing plural coils and performing aliasing in images is being put into practical use (conventional technique 1: J. B. Ra, C. Y. Rim: Fast Imaging Using Subencoding Data Sets from Multiple Detectors, MAGNETIC RESONANCE in Medicine, vol.30, pp.142-145 (1993)).
The conventional technique 1 involves a method of shortening the imaging time to 1/(sub-coil number) by using a reception coil having a plurality of sub-coils. For example, when two sub-coils are used, the desired field of view (FOV) is divided into halves along the phase-encoding direction, and the halves are respectively imaged by two coils at the same time, whereby the imaging time is cut in half. The two half FOV images include aliasing artifacts of the object. However, the artifacts are removed by solving simultaneous equations utilizing the different sensitivity distributions of the two coils, whereby a 1/1 FOV image is reconstructed.
FIGS. 10(a) and 10(b) are diagrams illustrating the conventional technique. A method of removing aliasing artifacts will be described with reference thereto. FIG. 10(a) shows a representative example of measurement data in phase space, or k-space (left side of FIG. 10(a)), and a real-space image (right side of FIG. 10(a)) when a desired FOV (one FOV image) is imaged using a conventional method with one coil. The measurement data of the k-space is subject to Fourier Transformation (F.T.) so as to be converted into a real-space image.
FIG. 10(b) shows a representative example of measurement data in k-space (left side of FIG. 10(b)) and a real-space image (right side of FIG. 10(b)), when imaging is performed using a method of shortening the imaging time by utilizing an array-coil having two sub-coils and aliasing artifacts of images. Both in FIG. 10(a) and FIG. 10(b), the x-direction indicates the readout direction, and the y-direction indicates the phase-encoding direction.
In the k-space measurement data in FIG. 10(b), the width of the interval between steps in the phase-encoding direction is twice as broad as that of the k-space measurement data in FIG. 10(a). When the k-space measurement data in FIG. 10(b) is subject to Fourier transformation, the FOV is narrowed in the y-direction to produce the real-space image in FIG. 10(b), and a portion of the object that does not fit in the FOV appears as aliasing artifacts. In the real-space image in FIG. 10(b), pixels at S, which is an overlap of aliasing artifacts, represent an overlap of the pixels located on A and B in the real space image in FIG. 10(a).
Here, in the image obtained by Fourier-transforming signals measured by the first of the two sub-coils, the signal intensity (luminance) of the pixel located at S is represented by VS1, and in the image obtained by Fourier-transforming signals measured by the second of the two sub-coils, the signal intensity of the pixel located at S is represented by VS2. The sensitivity of the first sub-coil to the pixel located at A with a standard value is represented by f1A, and its sensitivity to the pixel located at B with a standard value is represented by f1B. Similarly, the sensitivity of the second sub-coil to the pixel located at A with a standard value is represented by f2A, and its sensitivity to the pixel at B with a standard value is represented by f2B. Magnetization existing on the pixels located at A is represented by IA, and magnetization existing on the pixels located at B is represented by IB. At this time, the relation of formula (1) is valid.                                                                                                     V                  S1                                                                                                      V                  S2                                                                              =                                                                                                              f                                          1                      ⁢                      A                                                                                                            f                                          1                      ⁢                      B                                                                                                                                        f                                          2                      ⁢                      A                                                                                                            f                                          2                      ⁢                      B                                                                                                                ⁢                                                                                                      I                    A                                                                                                                    I                    B                                                                                                                      (        1        )            
A pre-scanning is performed so as to calculate the sensitivity distribution of plural sub-coils constituting the reception coil, and the elements of the matrix f1A, f1B, f2A, and f2B in the formula (1) are calculated in advance. Since VS1 and VS2 can be calculated, unknowns IA and IB are calculated by solving the matrix equation of formula (1).
The number of rows in the matrix corresponds to the number of sub-coils. For example, when the sub-coil number is eight, the number or rows in the matrix is eight. At the same time, the number of columns in the matrix has to be no more than the sub-coil number. For example, when the sub-coil number is eight, the number of columns in the matrix has to be eight or less. This is because the number of unknowns in the simultaneous equations must be no more than the number of equations.
FIG. 11 shows an example of the arrangement of a reception coil (array-coil) 118 as used in the conventional technique. The reception coil 118 includes a plurality of rectangular coils arranged on the surface of the object to be examined 103. The output of the reception coil 118 is sampled at a receiver 108 and is stored in the memory of a computer 109.
Usually, when two coils are placed close to each other, the resonance point of the input impedance of the coils is divided into two or more, due to mutual inductive coupling. Among the plurality of sub-coils constituting the reception coil 118, the adjacent sub-coils are usually overlapped moderately, as shown in FIG. 11, so as to eliminate the mutual inductive coupling. If the degree of overlapping is not appropriate, the resonance point of the input impedance of the coils is divided into two or more. By serially connecting a subsidiary coil to each of the adjacent coils, the inductive coupling between the adjacent coils can be eliminated without overlapping.
In a conventional imaging method, an image is generated only from positional information in the phase-encoding direction obtained by a gradient magnetic field. On the other hand, in the imaging method according to the conventional technique 1, the sensitivity distribution of the reception coil is actively utilized, along with the gradient magnetic field, in generating an image from the positional information in the phase-encoding direction. The imaging-time shortening technique, according to the conventional technique 1, is effective in speeding up all imaging methods from the usual imaging method to an ultra-fast imaging method.
To use the speed-up technique according to the conventional technique 1, one or more sub-coils having nonuniform sensitivity distributions in the phase-encoding direction of an MR image of an imaging section are required. By “nonuniform sensitivity distribution”, it is meant that, in the area of the object being examined, there is a point where the sensitivity of the coil is half the highest sensitivity.
FIGS. 8(a), 8(b), 8(c), and 8(d) show a loop coil 91-2 according to the present invention, the sensitivity distribution 81 thereof along the x-axis, the sensitivity distribution 82 thereof along the y-axis, and the sensitivity distribution 83 thereof along the z-axis, respectively. As shown in FIG. 8(a), the diameter of the loop coil 91-2 is 40 cm, and the loop coils 91-1 and 91-3 (not shown) have the same diameter. Among the sensitivity distributions along the respective axes, only the sensitivity distribution 82 on the y-axis has, within the area of the object, some points where the sensitivity is half or less than the highest sensitivity.
Generally, when the number of sub-coils having a nonuniform sensitivity profile in the phase-encoding direction, among the plurality of sub-coils constituting the reception coil, is n, the speed with which a desired FOV (1/1 FOV image) is imaged can be raised (n+1)-fold compared to that when one coil only is used, by using the conventional technique 1. In the following description, the improved ratio of imaging speed is referred to as an imaging speed-up ratio. However, when the imaging-speed raising ratio is represented by N, the sensitivity distributions of N or more sub-coils need to be different from each other.
In the formula (1), if the relation shown in the formula (2) is valid, the matrix in the formula (1) does not have an inverse matrix and the solution thereof is indefinite.f1A:f1B=f2A:f2B  (2) 
FIGS. 12(a) to 12(d) show examples of an imaging section and sensitivity distributions along certain lines. FIG. 13 shows an example of the arrangement of the loop coils according to the present invention. In FIG. 13, the z-axis corresponds to a static magnetic field direction. The loop coil 91-1, which is disposed on a plane intersecting with the xz plane at an angle of 45°, and the loop coil 91-3, which is disposed on a plane intersecting with the xz plane at an angle of −45°, are arranged on the outer periphery of the object 103. The imaging section 121 is set to the xy plane, and the FOV is set from −15 cm to 15 cm. As shown in FIG. 12(a), the imaging section is determined by the x-axis and the y-axis.
Here, the phase-encoding direction is set to the y-direction. FIG. 12(b) shows the sensitivity distributions 123-1 and 123-2 of the coils 91-1 and 91-2, respectively, on a line segment 122-1 parallel to the y-axis. FIG. 12(c) shows the sensitivity distributions 124-1 and 124-2 of the coils 91-1 and 91-2, respectively, on a line 122-2 parallel to the y-axis. FIG. 12(d) shows sensitivity distributions 125-1 and 125-2 of the coil 91-1 and 91-2, respectively, on a line 122-3 parallel to the y-axis.
As shown in FIG. 12(c), the sensitivity distributions 124-1 and 124-2 match each other. The sensitivity distributions 123-1 to 125-2 shown in FIGS. 12(b), 12(c), and 12(d) are nonuniform sensitivity distributions along the y-axis. When using the speed-up technique, the aliasing artifact on the line segments 122-1 and 122-3 can be removed by using the difference between the sensitivity distributions of the two coils. However, the aliasing artifact on the line segment 122-2 cannot be removed, since the sensitivity distributions of the two coils on the line segment 122-2 are the same.
Also, when the values of f1A/f1B and f2A/f2B are not completely the same, but are close to each other, that is, when the sensitivity distributions of the two sub-coils are similar, a problem occurs in that, when image reconstruction is performed, the S/N ratio of the image is deteriorated on the lines along which the sensitivity distributions of the two sub-coils are similar.
The speed-up according to the conventional technique 1 has been hitherto developed mainly for an apparatus of the horizontal magnetic field type with high magnetic field, and various structures of reception coils for use in the horizontal-magnetic-field-type apparatus have been proposed. On the other hand, an open MRI apparatus of the vertical magnetic field type using a superconductive magnet or a permanent magnet, has been drawing attention in recent years.
FIG. 3 is a perspective view of an open-type MRI apparatus according to the present invention. Since the open MRI apparatus of the vertical magnetic field type has a large opening, as shown in FIG. 3, a doctor 114 can directly access the object 103, whereby this apparatus is suitable for interventional MRI.
Since the direction of an RF (Radio Frequency) magnetic field generated by coils needs to be perpendicular to the direction of a static magnetic field, the structure of the reception coil also needs to be changed when the direction of the static magnetic field is changed from horizontal to perpendicular. The arrangement of the reception coil for the apparatus of a vertical magnetic field type also has been proposed (conventional technique 2: G. Randy Duensing, et. al.: A 4-channel Volume Coil for Vertical Field MRI, ISMRM, p1398 (2000)). However, in the structure proposed hitherto, there is a problem in that the imaging speed-up ratio can only be improved by twice when the phase-encoding direction is set to the static magnetic field direction.
The speed-up technique described in the conventional technique 1 is effective for imaging moving organs, such as the heart. To deal with fast movement of an organ, like the heart, the imaging speed-up ratio needs to be improved by three times or more.
However, among the four sub-coils mentioned in the conventional technique 2, only one sub-coil has a nonuniform sensitivity profile in the static magnetic field direction. Therefore, if such sub-coils are applied to the speed-up technique, the imaging speed-up ratio can be improved by three times or more by setting the phase-encoding direction to a direction different from the static magnetic field direction. However, when the phase-encoding direction is set to the static magnetic field direction, the imaging speed-up ratio can be improved by only twice.
That is, when imaging a transverse plane which is often used in the imaging of the heart, there is a problem in that the imaging speed cannot be sufficiently raised when the phase-encoding direction is set to a direction of chest-to-back (static magnetic field direction). The imaging speed can be raised by setting the phase-encoding direction to a direction other than the static magnetic field. However, in the imaging of the heart, there is a problem that the user's need for freely setting the phase-encoding direction in order to reduce artifacts of blood flow is not satisfied.
Also, an array-coil has been proposed which has figure-eight coils 117 as the sub-coils, as shown in FIG. 2 (conventional technique 3: Japanese Patent Laid-open Publication JP-A-04-282132). FIG. 4 shows RF current directions (arrows in the figure) on the figure-eight coils 117 according to the conventional technique. FIG. 5 shows RF magnetic fields generated by the figure-eight coils 117 shown in FIG. 4, wherein the arrows represent the direction of the RF magnetic fields and the size of the arrows represents the size of the RF magnetic fields. FIG. 6 shows the sensitivity of the figure-eight coils 117 shown in FIG. 4, wherein the size of the arrows represents the size of the sensitivity.
Among the RF magnetic fields shown in FIG. 5, only the component perpendicular to the static magnetic field contributes to the NMR phenomenon. Therefore, the distribution of sensitivity is generated as indicated by the arrows in FIG. 6. That is, the figure-eight coils 117 shown in FIG. 4 have a sensitivity above and below conductors 41 and 42, and the sensitivity profile thereof has a gradient in the y-direction. When a plurality of the figure-eight coils 117 arranged as shown in FIG. 2, an array-coil, including two or more sub-coils having nonuniform sensitivity profile on an arbitrary axis, can be formed.
However, since the figure-eight coils 117 have a sensitivity only above and below the conductors 41 and 42 in FIG. 4, there is a problem that the sensitivity of all sub-coils, except in the portion below the conductors 41 and 42, is significantly deteriorated when performing imaging on a coronal plane (xy plane) with the arrangement of the coils shown in FIG. 2. Consequently, there is an area in the FOV in which the coils do not have a sensitivity.
Therefore, the object of the present invention is to provide an MRI apparatus that is suitable for interventional MRI, in which selection of the imaging section and the phase-encoding axis is not limited.